The present invention is generally directed toward a surgical bone product and more specifically is a flowable gel and a malleable putty based on demineralized allograft bone particles mixed in a fluid carrier comprising a high molecular weight viscous excipient derived from the class of biomaterials known as hydrogels.
Malleable putty is used to correct surgical defects that may be caused by trauma, pathological disease, surgical intervention or other situations where defects need to be managed in osseous surgery. It is important to have the defect filler in the form of a stable, viscous putty to facilitate the placement of the bone growth medium into the surgical site which is usually uneven in shape and depth. The surgeon will take the putty on a spatula or other instrument and trowel it into the site or take it in his/her fingers to shape the bone inducing material into the proper configuration to fit the site being corrected.
Many products exist to treat this surgical need. One example is autologous bone particles or segments recovered from the patient. When removed from the patient, it is wet and viscous from the associated blood. This works very well to heal the defect but requires significant secondary surgery resulting in lengthening the surgery, extending the time the patient is under anesthesia and increasing the cost. In addition, a significant increase in patient morbidity is attendant in this technique as the surgeon must take bone from a non-involved site in the patient to recover sufficient healthy bone, marrow and blood to perform the defect filling surgery. This leads to significant post-operative pain.
Another product group involves the use of inorganic materials to provide a matrix for new bone to grow at the surgical site. These inorganic materials include hydroxyapatite obtained from sea coral or derived synthetically. Either form may be mixed with the patient""s blood and/or bone marrow to form a gel or a putty. Calcium sulfate or plaster of Paris may be mixed with water to similarly form a putty. These inorganic materials are osteoconductive but are bioinert and do not absorb or become remodeled into natural bone. They consequently remain in place indefinitely as a brittle, foreign body in the patient""s tissue.
Allograft bone is a logical substitute for autologous bone. It is readily available and precludes the surgical complications and patient morbidity associated with autologous bone as noted above. Allograft bone is essentially a collagen fiber reinforced hydroxyapatite matrix containing active bone morphogenic proteins (BMP) and can be provided in a sterile form. The demineralized form of allograft bone is naturally both osteoinductive and osteoconductive. The demineralized allograft bone tissue is fully incorporated in the patient""s tissue by a well established biological mechanism. It has been used for many years in bone surgery to fill the osseous defects previously discussed.
It is well known in the art that for several decades surgeons have used a patient""s own blood as a vehicle in which to mix the patient""s bone chips or bone powder, or demineralized bone powder so as to form a defect filling paste. Blood is a useful carrier because it is available from the bleeding operative site, is non-immunogenic to the patient and contains bone morphogenic proteins which facilitate wound healing through new bone growth. However, stored blood from other patients has the deficiencies that any blood transfusion would have such as blood type compatibility, possibility of transmission of disease and unknown concentration of BMP which are to a great extent dependent upon the age of the donor.
While blood contains from forty percent (40%) to fifty percent (50%) cell mass, it is a satisfactory carrier for demineralized bone powder because it contains both mono- and polysaccharides which contribute to the blood viscosity and provide the bulk viscosity to the paste created by mixing the bone powder and blood. Specific monosaccharides in blood are glucose at a concentration of 60-100 mg/100 ml (0.1%) and polysaccharides such as hexose and glucosamine at approximately 0.1%. Glucuronic acid is also present at approximately 0.4-1.4 mg/100 ml (average 0.01%).
The problems inherent with using the patients blood as a carrier for demineralized bone powder are the difficulties of mixing the same at the operating site, the difficulty in obtaining a bone paste consistency which can be easily applied to the surgical area, the guesswork in mixing a usable composition at the site and the problem of having a bone paste or gel which will promote optimum bone replacement growth, not be carried away by the body fluids at the operation site or simply fall out of the bone defect site. In an attempt to solve these and other problems, there have been a number of other attempts using other alternative mixtures and compositions.
Demineralized allograft bone is usually available in a lyophilized or freeze dried and sterile form to provide for extended shelf life. The bone in this form is usually very coarse and dry and is difficult to manipulate by the surgeon. One solution to use such freeze dried bone has been provided in the form of a gel. GRAFTON(copyright), a registered trademark of Osteotech Inc., which is a simple mixture of glycerol and lyophilized, demineralized bone powder of a particle size in the range of 0.1 cm to 1.2 cm (1000 microns to 12,000 microns) as is disclosed in U.S. Pat. No. 5,073,373.
GRAFTON works well to allow the surgeon to place the allograft bone material at the site. However, the carrier, glycerol has a very low molecular weight (92 Daltons) and is very soluble in water, the primary component of the blood which flows at the surgical site. Glycerol also experiences a marked reduction in viscosity when its temperature rises from room temperature (typically 22xc2x0 C. in an operating room) to the temperature of the patient""s tissue, typically 37xc2x0 C. This combination of high water solubility and reduced viscosity causes the allograft bone material to be xe2x80x9crunnyxe2x80x9d and to flow away from the site almost immediately after placement; this prevents the proper retention of the bone within the site as carefully placed by the surgeon.
These problems with GRAFTON gel have been attempted to be resolved by using a much larger particle size of allograft bone, specifically lamellae or slivers of bone created by milling or slicing the bone before mixing it with the glycerol carrier. This improves both the bulk viscosity and the handling characteristics of the mixture but still leaves the problem of the fast rate of dissipation of the carrier and some bone due to the solubility of the glycerol carrier. The larger particles of demineralized bone may also retard the development of new bone by the patient because the large bony lamellae do not pack as well as the smaller grainy particles of bone. This will leave more open space and could lengthen the tine required to grow new bone and properly fill the defect. Another deficiency of using the bone lamellae is that the ends of the bony fragments are uneven and when packed into the surgical defect, leave uneven filaments of bone protruding out from the defect which can compromise the healing rate.
U.S. Pat. No. 5,290,558 discloses a flowable demineralized bone powder composition using a osteogenic bone powder with large particle size ranging from about 0.1 to about 1.2 cm. mixed with a low molecular weight polyhydroxy compound possessing from 2 to about 18 carbons including a number of classes of different compounds such as monosaccharides, disaccharides, water dispersible oligosaccharides and polysaccharides.
Hence, the advantages of using the smaller bone particle sizes as disclosed in the U.S. Pat. No. 5,073,373 gel patent were compromised by using bone lamellae in the shape of threads or filaments and retaining the low molecular weight glycerol carrier. This later prior art is disclosed in U.S. Pat. Nos. 5,314,476 and 5,507,813 and the tissue forms described in these patents are known commercially as the GRAFTON Putty and Flex, respectively.
The use of the very low molecular weight glycerol carrier also requires a very high concentration of glycerol to be used to achieve the bulk viscosity. Glycerol and other similar low molecular weight organic solvents are toxic and irritating to the surrounding tissues. Furthermore glycerol has been reported to be specifically neurotoxic and this problem is compounded when the concentration of glycerol is at the 20-95% level as disclosed in the U.S. Pat. No. 5,073,373 patent.
Another attempt to solve the bone composition problem is shown in U.S. Pat. No. 4,172,128 which discloses demineralized bone material mixed with a carrier to reconstruct tooth or bone material by adding a mucopolysaccharide to a mineralized bone colloidal material. The composition is formed from a demineralized coarsely ground bone material, which may be derived from human bones and teeth, dissolved in a solvent forming a colloidal solution to which is added a physiologically inert polyhydroxy compound such as mucopolysaccharide or polyuronic acid in an amount which causes orientation when hydrogen ions or polyvalent metal ions are added to form a gel. The gel will be flowable at elevated temperatures above 35 C. and will solidify when brought down to body temperature. Example 25 of the patent notes that mucopolysaccharides produce pronounced ionotropic effects and that hyaluronic acid is particularly responsible for spatial cross-linking. Unfortunately this bone gel is difficult to manufacture and requires a premolded gel form.
U.S. Pat. No. 4,191,747 teaches a bone defect treatment with coarsely ground, denatured bone meal freed from fat and ground into powder. The bone meal is mixed with a polysaccharide in a solution of saline and applied to the bone defect site.
Another prior art product is the formulation of demineralized allograft bone particles in collagen. Both bovine and human collagen have been used for this application. Bovine collagen carries the risk of an immunogenic reaction by the recipient patient Recently, it has been found that a disease of cattle, bovine spongioform encephalopathy (BSE) is transmitted from bovine tissue to humans. Thus, bovine tissue carries a risk of disease transmission and is not a desirable carrier for allograft tissue.
Human collagen is free of these animal based diseases. However, collagen absorbs slowly in the human body, particularly in a bony site with usually a low degree of vascularity. The slow absorption of collagen can delay the growth of new bone and result in the formation of scar tissue at the site. This could result in a non-bony healing and a result with much less tensile strength.
Accordingly, the prior art as embodied in the glycerol and other carrier base technology to deliver demineralized allograft bone to a surgical osseous site is replete with problems and only partially addresses the problems inherent in the correcting surgical defects.
A bone putty with a useful bulk viscosity has been achieved by using a very high molecular weight class of soluble biomaterial, hydrogel. The use of high molecular weight hydrogels preferably over one million Daltons allows the achievement of a very malleable bone putty with only 1-3% concentration of the hydrogel in the carrier. The balance of the carrier formulation is a sterile saline or pure water which avoids the toxic problems with the high concentrations of the low molecular weight organic solvents of the prior art.
It can thus be seen that the prior art has attempted to replicate putty/gel obtained by the mixing of blood with bone particles without the necessity of mixing the two together at the surgical site in non-controlled proportions and under time and space prohibitions.
The selection of high molecular weight hydrogels allows the use of the preferred small particle size granules of demineralized allograft bone. These small particles pack better in the wound defect and absorb more quickly thereby allowing the bone defect to be remodeled into the natural bone of the patient.
It is an object of the invention to utilize demineralized powdered bone in a particle size that is useful to achieve the malleability characteristics that maximizes the amount of bone in the formulation without creating a gritty, less malleable characteristic.
It is yet another object of the invention to use a calcium salt with the demineralized bone composition to aid in healing at the bone defect site.
It is an additional object of the invention to use a non toxic carrier for the bone particles which will not adversely impact on the patient.
It is another object of the invention to provide a premixed bone putty/gel in an oxygen protected carrier to keep the putty/gel from drying out or being degraded.
It is also an object of the invention to create a bone defect material which can be easily handled by the physician and does not degenerate when contacting blood flow at the surgical site.
The present invention is directed towards a demineralized bone powder composition to heal bone defects. The preferred embodiment of Examples I and VIII are the best mode for the putty composition and Examples XV or XVI for the gel composition. These and other alternate embodiments of the invention overcome the two basic deficiencies of the glycerol carrier and bone particle flowable compositions used in the prior art: first, the low molecular weight of glycerol; and second, the use of large particle or lamellae to achieve the preferred bulk viscosity. The types of demineralized bone used in the invention are cortical and cortico-cancellous bone powder.
Surprisingly, the combination of the 100-420 micron particle size of demineralized, lyophilized, allograft bone when mixed with very low concentrations of these very high molecular weight hydrogels in a suitable carrier produces a malleable putty with clinically useful bone inducing properties. The malleable property permits the surgeon to shape the quantity of bone putty or gel to exactly fit the surgical defect. Manipulation of the xe2x80x9clumpxe2x80x9d of bone putty may be done without it sticking to the gloves of the surgeon, behaving somewhat like a wet clay used in sculpting.
The ideal carriers for the malleable putty are preferably taken from high molecular weight hydrogels such as 1) Sodium Hyaluronate about 7.0xc3x97105-3.0xc3x97106 Daltons; 2) Chitosan about 1.0xc3x97105-3.0xc3x97105 Daltons; 3) Dextran about 1.0xc3x97103-1.0xc3x97105 Daltons; and 4) Pluronics about 7.0xc3x97103-1.8xc3x97104 Daltons; and 5) N,O-carboxymethylchitosan glucosamine (NOCC) which is an example of the class of hydrogels known as glycosaminoglycan, a hydrogel derivative about 2.0xc3x97106-3.0xc3x97106 Daltons .
The molecular weight of the hydrogels used in the carriers set forth in the Examples I-XVII are: Hyaluronic acidxe2x80x94(1.2xc3x97106 Daltons). Chitosanxe2x80x94(2.0xc3x97105 Daltons). Dextran (40,000 Daltons, used in example VII) or the Pluronic block copolymers of polyethylene oxide and polypropylene oxide; Pluronic(copyright) F127 - 9849 to 14,600 Daltons (avg. mol. wt.: 12,600 Daltons); Pluronic(copyright) F108-12,700 to 17,400 Daltons (avg. mol. wt.: 14,600 Daltons).
Demineralized, lyophilized allograft bone of particle size of about 100 to about 420 microns at a concentration of about 30% to 35% w/w is mixed into an isotonic saline solution of 2% hyaluronic acid of an average molecular weight of about 1.2 million Daltons and produces a highly desirable malleable bone putty. Hyaluronic acid is generally described as an acid mucopolysaccharide. It is envisioned that suitable amounts of bone morphogenic proteins (BMP) can be added to either the gel or putty at any stage in the mixing process to induce accelerated healing at the bone site. BMP directs the differentiation of pluripotential mesenchymal cells into osteoprogenitor cells which form osteoblasts. The ability of freeze dried demineralized cortical bone to transfer this bone induction principle using BMP present in the bone is well known in the art. However the amount of BMP varies in the bone depending on the age of the bone donor and the bone processing. Sterilization is an additional problem is processing human bone for medical use as boiling, autoclaving and irradiation over 2.0 mrads is sufficient to destroy or alter the BMP present in the bone matrix.
Another embodiment of the invention is to induce the presence of soluble calcium at the bone defect site. This will encourage new bone growth through the normal biochemical mechanism. Soluble calcium can be attracted to the surgical site by using a sodium phosphate buffer of pH 7.2 in lieu of the isotonic saline. The phosphate buffer will attract calcium cations to the site from the surrounding healthy bone and create an equilibrium concentration of the calcium precisely at the site of healing where it is most desirable to grow new bone.
Another embodiment of the invention is to create a sponge sheet or sponge mat of bone which is flexible and can be cut to shape by the surgeon. This can be made by using a cross linked hydrogel, either hyaluronic acid or chitosan and suspending a high concentration of bone particles ranging from 250-850 microns in size with up to 75% bone by weight. This is then lyophilized or freeze dried to remove the water component via ice sublimation leaving behind a flexible sheet of bone suspended in the dehydrated hydrogel matrix.
Any number of medically useful substances can be used in the invention by adding the substances to the composition at any steps in the mixing process or directly to the final composition. Such substances include collagen and insoluble collagen derivatives, hydroxy apatite and soluble solids and/or liquids dissolved therein. Also included are antiviricides such as those effective against HIV and hepatitis; antimicrobial and/or antibiotics such as erythromycin, bacitracin, neomycin, penicillin, polymyxin B, tetracycline, viomycin, chloromycetin and streptomycin, cefazolin, ampicillin, azactam, tobramycin, clindamycin and gentamycin. It is also envisioned that amino acids, peptides, vitamins, co-factors for protein synthesis; hormones; endocrine tissue or tissue fragments; synthesizers; enzymes such as collagenase, peptidases, oxidases; polymer cell scaffolds with parenchymal cells; angiogenic drugs and polymeric carriers containing such drugs; collagen lattices; biocompatible surface active agents, antigenic agents; cytoskeletal agents; cartilage fragments, living cells such as chondrocytes, bone marrow cells, mesenchymal stem cells, natural extracts, tissue transplants, bioadhesives, transforming growth factor (TGF-beta), insulin-like growth factor (IGF-1): growth hormones such as somatotropin; bone digestors; antitumor agents; fibronectin; cellular attractants and attachment agents; immuno-suppressants; permeation enhancers, e.g. fatty acid esters such as laureate, myristate and stearate monoesters of polyethylene glycol, enamine derivatives, alpha-keto aldehydes can be added to the composition.